In a typical base station of the prior art, local oscillator signals are provided for each one of the transceivers in the base station. Likewise, in a remote radio head application, individual local oscillator signals are also provided individually for each one of the transceivers located in the remote radio head application. It is necessary to provide multiple numbers of individual local oscillator signals, since each one of the transceivers may be operating on different channels. Multiple local oscillators may also be included to improve reliability through the removal of the single point of failure which a single local oscillator would provide.
One issue associated with the approach of utilizing a multiple number of individual local oscillators is the expense and real estate on a chip associated with providing the plurality of local oscillators and the possible need to calibrate the different ones of the oscillators. This can be an issue during a start-up phase. For example, if the individual local oscillators are not correctly calibrated at the start-up, this may lead to difficulties in ensuring that the required beam forming operations for the radio signals are undertaken correctly. In particular, this may mean that the correct beam shapes for the radio signal in the required directions are not correctly calculated.
FIG. 1 shows an example of an active antenna array as known in the prior art and comprising a plurality of transmission paths. Only a first signal path at the top, a second signal path in the middle and a last or n'th signal path at the bottom are illustrated in FIG. 1 (as well as in the subsequent figures). The third to the (n−1)th transmission paths are not illustrated for the sake of clarity.
A radio signal in the digital domain to be transmitted reaches the active antenna array from the left and is fed to the digital signal processor 15. The digital signal processor 15 distributes the radio signals to be transmitted to a plurality of output paths 16a, 16b, . . . , 16n. In the prior art example illustrated the radio signals to be transmitted by the plurality of output paths 16a, 16b, . . . , 16n are digital IF transmission signals which have undergone upconversion in the digital signal processor 15. Other processes may also take place in digital signal processor 15 and these include, but are not limited to: crest factor reduction, digital predistortion and digital beamforming. The inclusion or omission of these processes has no impact on the teachings of the disclosure as described herein. For simplicity the letters relating to all of the paths will be left out in future reference numerals.
Only the passage of the transmission signal through the top one of the output paths 16a will be described in detail. It will be appreciated that all of the other output paths 16b, . . . , 16n are identical. The output path 16a is connected to a digital-to-analogue converter 20a which converts the digital IF transmission signals from the digital signal processor 15 to analogue signals prior to passing the analogue signals through a first filter 25a to obtain those filtered transmission signals in the desired frequency band. The filtered transmission signals in the desired frequency band are forwarded to a first mixer 30a. The first mixer 30a upconverts the filtered transmission signals by means of a first oscillator 35a to an analogue intermediate frequency band. The first oscillator 35a is clocked by a signal from a first reference clock 100a. 
The output of the first mixer 30a is filtered in a second filter 40a and passed to an intermediate frequency amplifier 45a. The output of the intermediate frequency amplifier 45a is passed to a second mixer 50a at which it is upconverted with an oscillator signal from the second local oscillator 55a. The second local oscillator 55a is also clocked from the first reference clock 100a. 
The transmission signals from the first mixer 50a are now at a transmission frequency band (the radio frequency) and are passed through a third filter 60a into a radio frequency amplifier 65a before entering a transmission filter 70a and being passed to the radio frequency output 80a. The radio frequency output 80a is connected to one of the plurality of antenna elements from the antenna array (not shown). A tap 75a provides a feedback loop to the digital signal processor 15 through paths 85 which allow calibration and updating of the predistortion processing, of the radio signals to be taken into account.